The invention relates to a heat-curable binder mixture comprising an organic synthetic resin A and a crosslinking agent B.
Various chemical reactions have been proposed and are indeed used for curing the binders in paint films by crosslinking. The chemical bonds formed in the course of the crosslinking reaction frequently do not meet all the demands which are made on the paint films.
Electropainting has in recent years become widely established as a method of priming electrically conductive substrates. Interest has hitherto concentrated on anodic electropainting. The resin binders used for this purpose belong to the carboxyl-containing resins, for example to the maleate oils, maleated epoxy resins, alkyd resins, acrylic resins and in particular to the maleated polybutadienes. These resins were water-solubilized by salt formation, chiefly with amines, and were electrodeposited in the electrocoating bath at the anode. The anodic electropriming process, however, has serious disadvantages. For instance, oxygen evolves at the anode in the course of the electrodeposition and can have a serious, adverse effect on the resins being deposited at the anode. Furthermore, metal ions pass into solution at the anode and end up as flaws in the baked film. The metal ions can be the cause of discoloration and spotting. They are in particular responsible for qualitative disadvantages as a result of salt formation, which reduces the resistance to water and the level of corrosion protection.
The cathodic electropriming process, which has come of age, commercially, in recent years, is increasingly displacing the anodic process, since the deficiencies described above are largely avoided. For instance, the gas which evolves at the cathode--where the paint film is now deposited--is hydrogen, which has no effect on the resin binder. Since cathodic deposition can take place at approximately neutral pH virtually no metal ions go into solution. The binders suitable for cathodic deposition mainly contain amino groups, which are neutralized with acids to achieve water-solubilization.